Experience with bilateral petrosal sinus sampling for the localization of ACTH secreting pituitary microadenomas has been extensively broadened. Over 150 patients have now been sampled and the results of surgery support the accurate localization provided by this technique. Patients with true microadenomas (less than 2 mm) have been cured by hemipituitary resection based on sampling results. In addition, thyrotropin secreting and prolactin secreting microadenomas have been successfully localized. Significance of Research: Our surgical results for pituitary microadenectomy in Cushing's disease are now better than other institutions by virtue of the fact that microadenomas too small to be detected by the operating surgeon, can be confidently removed by performing a blind hemiresection on the basis of sampling data. Another advantage of petrosal sinus sampling in Cushing's disease is that accumulating evidence suggests it is now the most accurate test for distinguishing the various causes of hypercortisolism. Extensive endocrinologic tests involving dexamethasone suppression, Metyrapone stimulation, CRH stimulation, etc., are used to distinguish pituitary versus ectopic ACTH versus adrenal causes of hypercortisolism, as well as the often confusing syndrome of pseudo-Cushing's disease. The petrosal sinus sampling examination is the single most accurate test both for establishing the presence of the disease, its etiology, and for lateralizing the adenoma.